1. Field of the Invention
The present invention relates to a linear encoder.
2. Description of the Related Art
A linear encoder mounted in a machine tool or the like is an example of a measurement device. First, a general structure of a linear encoder will be described. FIG. 1 is a schematic diagram illustrating a general linear encoder. FIG. 6 is a sectional view of the general linear encoder. In the following description, the longitudinal direction, the transverse direction, and the thickness direction of a scale included in the linear encoder are referred to as the X-axis direction, the Y-axis direction, and the Z-axis direction, respectively, for convenience (see FIG. 6).
As illustrated in FIGS. 1 and 6, in a general linear encoder, a scale 2 is held in a frame 1, and an opening 1a is formed in the frame 1 so as to extend in the X-axis direction. The scale 2 and a detector 3 are moved relative to each other while the detector 3 is inserted through the opening 1a, and the detector 3 obtains position information from the scale 2 (see, for example, Japanese Unexamined Patent Application Publication No. 2008-267989).
In this structure, to prevent oil, dust including machining dust, etc., from entering the frame 1 through the opening 1a, the frame 1 is provided with a first dustproof member 101 and a second dustproof member 102 arranged to face each other across the opening 1a in the Z-axis direction. The first dustproof member 101 and the second dustproof member 102 are disposed with the detector 3 interposed therebetween. When the scale 2 and the detector 3 are moved relative to each other, the detector 3 moves along the opening 1a of the frame 1 in the X-axis direction while thrusting the first dustproof member 101 and the second dustproof member 102 aside.
The structure of each of the first dustproof member 101 and the second dustproof member 102 will now be described. FIG. 7 illustrates a structure of the first dustproof member 101 before it is deformed. FIG. 8 illustrates the manner in which the first dustproof member 101 and the second dustproof member 102 cover the opening 1a of the frame 1 in a region where the detector 3 is not inserted through the opening 1a. FIG. 9 illustrates the manner in which the first dustproof member 101 and the second dustproof member 102 cover the opening 1a of the frame 1 in a region where the detector 3 is inserted through the opening 1a. 
The first dustproof member is made of a flexible material. The length of the first dustproof member 101 in the X-axis direction is substantially equal to the length of the opening 1a of the frame 1 in the X-axis direction. As illustrated in FIG. 7, the first dustproof member 101 includes a base portion 101a, a tongue portion 101b, and a projecting portion 101c. 
The base portion 101a is fitted in a first fitting recess 1b1. In order that the base portion 101a can be easily inserted into the first fitting recess 1b1 of the frame 1, the base portion 101a has a size that is smaller than that of the first fitting recess 1b1 of the frame 1 when viewed in the X-axis direction. Therefore, the base portion 101a and the first fitting recess 1b1 of the frame 1 are not in close contact with each other, and a gap is easily formed between the base portion 101a and the first fitting recess 1b1 of the frame 1.
The tongue portion 101b has a plate-like shape and projects from the base portion 101a. When the base portion 101a is fitted in the first fitting recess 1b1 of the frame the tongue portion 101b projects from the opening of the first fitting recess 1b1.
The projecting portion 101c is formed on a surface of the tongue portion 101b facing in the positive Y-axis direction. The projecting portion 101c has a substantially right triangular shape when viewed, in the X-axis direction. A surface of the projecting portion 101c near the tip of the first dustproof member 101 is substantially perpendicular to the tongue portion 101b. 
The second dustproof member 102 will not be described in detail because its structure is similar to that of the first dustproof member 101. The second dustproof member 102 includes a base portion 102a fitted in a second fitting recess 1b2 of the frame 1, a tongue portion 102b projecting from the base portion 102a, and a projecting portion 102c formed on a side of the tongue portion 102b facing in the positive Y-axis direction. When the base portion 102a is fitted in the second fitting recess 1b2 of the frame 1, the tongue portion 102b projects through the opening of the second fitting recess 1b2.
The first fitting recess 1b1 and the second fitting recess 1b2 are formed in the frame 1 so as to face each other in the Z-axis direction with the opening 1a of the frame 1 therebetween. The length of the first fitting recess 1b1 and the length of the second fitting recess 1b2 in the X-axis direction are each substantially equal to the length of the opening 1a in the X-axis direction.
In the region where the detector 3 is not inserted through the opening 1a of the frame 1, as illustrated in FIG. 8, the tongue portion 101b of the first dustproof member 101 and the tongue portion 102b of the second dustproof member 102 contact each other to close the opening 1a of the frame 1 in such a state that the first dustproof member 101 and the second dustproof member 102 are warped.
At this time, for example, the tongue portion 102b of the second dustproof member 102 is engaged with the projecting portion 101c of the first dustproof member 101, and the tongue portion 101b of the first dustproof member 101 is engaged with the tip of the tongue portion 102b of the second dustproof member 102. Thus, even when the first dustproof member 101 or the second dustproof member 102 tries to rotate in the negative Y-axis direction, the first dustproof member 101 or the second dustproof member 102 is restrained by the restoring force thereof.
In the region where the detector 3 is inserted through the opening 1a of the frame 1, as illustrated in FIG. 9, the tongue portion 101b of the first dustproof member 101 and the tongue portion 102b of the second dustproof member 102 sandwich the detector 3 in such a state that the first dustproof member 101 and the second dustproof member 102 are warped. At this time, the projecting portion 101c of the tongue portion 101b of the first dustproof member 101 and the projecting portion 102c of the tongue portion 102b the second dustproof member 102 contact the detector 3.
In the dustproof structure of a general linear encoder, as described above, gaps are likely to be formed between the base portion 101a of the first dustproof member 101 and the first fitting recess 1b1 of the frame 1 and between the base portion 102a of the second dustproof member 102 and the second rioting recess 1b2 of the frame 1. Therefore, dust and the like are likely to enter the frame 1 from the outside.